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Old 09-16-2008, 07:03 PM   #1
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help with roof venting


hello all,

i am redoing the shingles on my roof this month. I have been going crazy on researching on the venting. My main question for the pros out there is "in your opinion are ridge vents or the power vents better?
The roof is two hip roofs joined(like an "L") and is vented trough the soffits .It is about 1300 square feet on a pich of about 4 to 8/12.

my main concern is will the ridge vent be able to breathe when it gets covered by the heavy snow ?

also which one of the ridge vents would you recommend for canadian winters (ontario)......
- shingle vent II
- cobra snow country advanced
- or the duraflo's

thanks a million for your opinion

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Old 09-16-2008, 08:52 PM   #2
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help with roof venting


Quote:
Originally Posted by NGTPHANTOM View Post
hello all,

i am redoing the shingles on my roof this month. I have been going crazy on researching on the venting. My main question for the pros out there is "in your opinion are ridge vents or the power vents better?
The roof is two hip roofs joined(like an "L") and is vented trough the soffits .It is about 1300 square feet on a pich of about 4 to 8/12.

my main concern is will the ridge vent be able to breathe when it gets covered by the heavy snow ?

also which one of the ridge vents would you recommend for canadian winters (ontario)......
- shingle vent II
- cobra snow country advanced
- or the duraflo's

thanks a million for your opinion
I read somewhere, that the Canada Research Counsel found that ventilation has little value in improving roof longevity, so perhaps you shouldn't worry about about venting too much.
There's a rule of thumb regards to sizing, its based on the how many square feet are to be vented.
If you have a concern about snow blockage, perhaps a louvered tower would be the way to go!

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Old 09-16-2008, 10:45 PM   #3
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help with roof venting


Hello,

The ridge vent will breathe if it is properly installed and the intake and exhaust guides are followed. I live in Alaska and we get an average of 75+'' of snow a year here and if done correctly there is no problems.
Ed the roofer would be the person to ask about the ventilation issues though.



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Old 09-17-2008, 12:12 AM   #4
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help with roof venting


See the photos and click to expand them in this thread I posted that contains Photos Of Ridge Vents and Hip Vent on the day after a heavy snowfall this past winter.

http://www.contractortalk.com/showth...ighlight=vents

Canadian source quoted the American test on 4 geographic locations and had dubious interpretations of the results.

But, Intake Ventilation is even more required than exhaust ventilation, with a desired NFVA balance being 60% Intake and 40% Exhaust.

Ed
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Old 09-17-2008, 11:47 AM   #5
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help with roof venting


thanks a lot for the replies. your pics ED were worth a thousand words and has helped making my decision a little easier. One thing i noticed on your pic was the hip vents. Would you recomend those on my style of roof ? I do have enough linear feet of ridge vent to acomodate the recomended exhaust for my roof, so will this just be overkill? Thanks
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Old 09-17-2008, 11:56 AM   #6
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help with roof venting


I only installed the Hip Vents because the entire lower roof was a vaulted ceiling and needed to vent each individual rafter bay.

The previous roof died after only 6-8 years due to no ventilation.

I also have the Smart Vent, from DCI Products installed at every one of the eave edges in those photos.

Did you notice how much retained heat was in the attic of the other roofs that I did not do, that were neighbors with 12/12 pitch roofs and also, even though they were 12/12's, how much ice damming was forming?

You seem to think that your ridges will be enough, so as long as you calculated it correctly, accept that as good enough. Typically, venting hips is a no-no, but each roof and attic needs to be analyzed on it's own merits and potential consequences.

Ed
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Old 09-17-2008, 08:14 PM   #7
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help with roof venting


thanks again Ed, i have based the calculations on the 1:300 ratio which i was advised instead of the 1:150 since i will be doing the intake via the continuous perforated soffit vent. I figured i needed about 17 feet , but i will be using the entire ridge which is about 26 feet in total .(one is 22 and the ataching ridge on "L" shaped roof is about 4.) I went into my roof today and noticed they only have 4 raffter baffles up there!. I checked it out and insulation is covering all the other openings.This is gonna be a pain to clear out all other soffit vents and install baffles.(got about 39 feet of perforated on the outside).
Would u happen to know the net free area/ foot that a 14" wide perforated aluminum soffit vent will give me? Also i checked the pitch and i measured it to be around 4 or 5/12. With such a low pitch, if it was u would u still consider the "shingle vent II" or would you feel better off using the louvers? i have attached a sketch of my roof style. Thanks a million!
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help with roof venting-roof-pic2.jpg  
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Old 09-17-2008, 10:55 PM   #8
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Each square foot of lanced or perforated aluminum soffit provides approximately between 6 square inches to 9 square inches of NFVA.

That assumes the wood soffit was cut out to the entire size of the vented covering panel, which typically is not done on a retro-fit installation.

That also does not include dust particles which gradually increase and negate the efficiency of the vented panels too.

Obviously, but not to everyone from many years of evidence, this also assumes that the vented panels are not painted shut.

Yes, a 5/12 is not any big deal at all for the Shingle Vent II and I install it on every pitch imaginable, from less than desired, with pre-cautionary measures taken and installed.

Roof Louvres, aka, mushroom vents only provide a maximum of 50-65 square incehs per vent, presuming that the vent hole is cut out to the proper size and specifcation. The amount you would need approximately to equal just the rated NFVA of the Shingle Vent II would be around 11 of the mushroom vents.

But, the external baffle on the Shingle Vent II creates additional suction outward from as little as a 3 mph wind flowing over it. So, in reality, it's actual performance is dramatically enhanced from most mild low windage days, which can only be guessed at, since wind speed is not consisantly static.

Ed
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Old 09-18-2008, 09:22 PM   #9
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thanks a lot Ed, you've been an amazing help! I have finally decided to do the shingle vent II . It seems to be worth the money and the design seems great. Thanks again for the great advice!!
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Old 09-18-2008, 10:27 PM   #10
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help with roof venting


You are welcome and I was glad do provide useful information that was easy to follow.

Ed
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Old 07-17-2011, 04:35 PM   #11
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help with roof venting


8/12 pitch hip roof, (covering a 40wide x34deep building) making for a short ridge with 2 lower gables and lower ridge above family room and 3 car garage hip.
roofer says I need both ridge and mushroom vents . plans on installing 6-8 vents one foot below the ridge vent . does this sound right to you?
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Old 07-18-2011, 12:24 AM   #12
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Quote:
Originally Posted by annblue View Post
8/12 pitch hip roof, (covering a 40wide x34deep building) making for a short ridge with 2 lower gables and lower ridge above family room and 3 car garage hip.
roofer says I need both ridge and mushroom vents . plans on installing 6-8 vents one foot below the ridge vent . does this sound right to you?

There are also vents made to be installed on the Hip Lines of your style home. www.AirVent.com manufactures one suct type of product.

I would prefer that to adding Mushroom Style vents, but even more importantly, how much Intake Ventilation do you have and is it freer flowing or blocked?

Ed

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